(1) Field of the Invention
The present invention relates to a sheet re-feeding device which first collects sheets having an image formed thereon and sends out the collected sheet again to the image forming station in order to effect duplex printing of the undersurface of, or superimposition printing of the same surface of, the sheet having an image formed thereon.
(2) Description of the Prior Art
Conventionally, it has been well known that duplex copy print can be obtained by a duplication technique, such as an electrophotographic process etc. This duplex printing of images on both sides of a sheet is performed by a process comprising the steps of: forming a static latent image on the photoreceptor and developing it into a toner image; transferring the toner image to a sheet of paper and fixing it on the sheet; collecting the sheet with the image formed on the first side into the intermediate tray; forming another static latent image on the photoreceptor and developing it into a toner image; and transferring the toner image to the second side of the sheet and fixing it on the sheet.
In an automatic duplex printing copier, when duplex printing of images are formed as above, a desired number of sheets with images printed on the first side thereof are collected on an intermediate tray, then the sheets on the intermediate tray are delivered out sheet by sheet while an image to be printed on the second side of the sheet is being formed on the photoreceptor so that the image is transferred to the second side of the sheet. This is a known method of duplex image forming for a multiple number of sheets, and is called a multi-copying method.
On the other hand, there is another method for duplex printing. This method is performed by forming an image on the first side of a sheet of paper, then collecting the sheet into an intermediate tray and directly re-feeding the sheet with one side printed so as to form another image on the second side. This operation will be repeated as required to obtain a multiple number of duplex copies. This method is known as a sequential copying method.
As an example of an automatic duplex image forming apparatus using the multi-copying method, it has been disclosed in Japanese Patent Application Laid-Open Sho 57 No.72,537, that in order to carry out duplex copying, when a multiple number of sheets are collected on the duplex printing sheet receptacle, the sheets are pressed from above whenever a sheet is stacked thereon in a way that makes it possible to perform stable sheet feeding even when curled sheets are stacked therein.
Further, it is disclosed in Japanese Patent Application Laid-Open Sho 61 No.140,425 that a sheet entering the tray of a duplex copying tray device is alternately pressed down at its rear end by a rear end presser and at its front end by a front end presser to correct rear and front end curls respectively, so as to prevent a following sheet from clashing with the sheets inside the tray and hence causing jamming.
In these conventional techniques, a sheet inverting mechanism was provided in such a manner that when a sheet is inverted and enters the intermediate tray to print an image on the second side of the sheet, the sheet is conveyed in a direction across that in which the sheet is conveyed through the transfer portion. This geometry made the apparatus large.
Specifically, in conventional configurations, the sheet which underwent the first copying operation tends to be set curled by the heating or pressing process of the fixing device and the inverting portion provided on its downstream side with respect to the sheet conveying direction. When such a sheet was accommodated in the duplex printing tray unit, the sheet would become bent or curled cylindrically resulting in improper accommodation, and hence the sheet could not be fed correctly for the second copying operation.
Referring further to the problems of the conventional art, in an image forming apparatus, when a sheet having an image formed thereon is input into the intermediate tray, the sheet often becomes curled across its width or length. The degree of curling is not equal and depends upon the individual sheets. Therefore, the front or rear end of an already accommodated sheet in the intermediate tray frequently snags or catches the front or rear end of a following sheet, causing sheet jams.
As means for solving the above problem, a protection device has been proposed in Japanese Patent Publication Hei 5 No.50,433. In this disclosure, in order to prevent the front end of a sheet being input to the intermediate tray from being snagged by the rear end of the already collected sheet in the intermediate tray, the rear end of the collected sheet is adapted to be pressed down from above. Then, at the timing when the front end of the sheet reaches the front end of the intermediate tray with respect to the sheet feeding direction, the front end of the already collected sheet is adapted to be pressed down, thus preventing the currently input sheet from being snagged with the already collected sheet and achieving a smooth input of sheets to the intermediate tray.
Therefore, the mechanism for pressing down the front and rear ends of sheets from above in the intermediate tray, disclosed in Japanese Patent Publication Hei 5 No.50,433, is constructed of a pair of front and rear end curl pressers which alternately move up and down.
In an image forming apparatus, in order to first collect a sheet having an image formed thereon and re-feed the collected sheet to the image forming station, it is necessary to solve the problem of sheet curling. However, in the mechanism disclosed in Japanese Patent Publication Hei 5 No.50,433, the device for inputting sheets into the intermediate tray becomes bulky. More specifically, before the front end of the sheet enters the intermediate tray, the entrance of the sheet should be detected so as to operate the rear end curl presser member, and then, before the front end of the sheet reaches the front end of the intermediate tray with respect to the sheet feeding direction, the state of the sheet should be detected so as to operate the front end curl presser member. In this way, this device needs a very bulky and complicated configuration for its control and mechanism and its controller circuit etc. Therefore, this configuration is extremely high in cost as a sheet re-feeding device containing an intermediate tray.
Further, the sheets once collected in the intermediate tray will cause improper sheet feeding when they are re-fed therefrom due to their differences in the curl direction and degree of curl.
For example, as shown in FIG. 1, a sheet P having an image formed thereon is conveyed by means of a conveyer roller 61 provided in the conveyance path 60 into an intermediate tray 62 disposed in the lower portion. When the front end of sheet P abuts an unillustrated limiting plate in intermediate tray 62, the conveyance of the rear end of the sheet by conveyer roller 61 completes and sheet P will be stacked due to gravity onto intermediate tray 62. At this moment, if the rear end of sheet P is curled, a presser member 63 located thereabove is rotated about an axle 63a toward the intermediate tray to press the rear end of curled sheet P down to the placement face of intermediate tray 62.
In the figure, 64 designates a sheet feed roller for feeding the lowermost one of stacked sheets P; 65 a delivering roller for delivering the sheet fed by sheet feed roller 64 to conveyance path 67 which is connected to the image forming station; 66 a separation roller which stops the delivery of the upper sheets except the sheet to be delivered by the delivering roller to achieve a single sheet delivery; and 68 and 69 designate respectively an actuator that operates under sheets P stacked on intermediate tray 62 and an optical sensor, constituting a sheet detecting means for detecting the existence of sheets.
After presser member 63 presses down the curled sheet, the presser member 63 should be reverted back to an unillustrated upper position in order to receive the subsequent sheet. At this moment, if the sheet has a strong rigidity and has been strongly curled, the sheet tends to revert back to the curled state or the rear end of the sheet pops up to a position where the front end of a subsequent sheet P will be conveyed by conveyer roller 61 and hence the curled sheet will snag the front end of sheet P being conveyed, causing a sheet jam when it is input into intermediate tray 62.
Further, even if the sheet is pressed down by sheet presser member 63, the rear end of the sheet might snag the slanted sheet guide surface as shown in FIG. 1. If the sheet in this state is re-fed in the opposite direction by sheet feed roller 64 and delivering roller 65, a delivery failure will occur.
Moreover, as the stacked number of sheets P in intermediate tray 62 is increased, the bottom becomes raised in proportion to the thickness of the sheets, the rear end of the sheets rises to a position where they hinder a following sheet P entering intermediate tray 62 even when the rear end of the sheets accommodated in intermediate tray 62 are pressed down by presser member 63. Therefore, it is necessary to limit the number of sheets held in intermediate tray 62.
As shown in FIG. 2, when a plurality of sheets P are accommodated being stacked on intermediate sheet tray 62, the degree of curling will be different in individual sheets. In such a case, a subsequent sheet P being input into intermediate tray 62 will push a previously accommodated sheet P1 toward a sheet stopper member 70, failing to align the front ends of the sheets flush with each other. Therefore, the rear ends of the sheets are displaced proportionally from the position of sheet feed roller 64, causing sheet delivery failure when a sheet is re-fed.